306 OUR HOUSEHOLD INSECTS 



incessant vibration whenever the insect moves, can be 

 seen from above, projecting forwards from the front 

 margin of the head, the real point of attachment 

 being, however, below, on the secondary jaws or 

 maxillsB. A strong pair of short, stout, primary jaws 

 (mandibles) constitute the only weapons of offence, 

 and these, as having a greater thickness of chitinous 

 covering, are darker coloured than any other part of 

 the body. Their shape cannot be seen without dis- 

 section, and this is no easy matter with such minute 

 creatures ; they can, however, be easily separated from 

 their attachments by the somewhat rough-and-ready 

 method of crushing the delicate little being between 

 two surfaces of glass ; on running in a drop or two of 

 water to clear away the fragments of soft tissues, they 

 may be isolated, and their outline 

 can then be readily traced. Fig. 104 

 shows their form, regularly curved 

 on the outer edge and toothed on 

 the inner. They work across one 

 another like the blades of a pair of 

 scissors. 



The thorax is the narrowest part 

 of the animal. The usual three divi- 

 FIG. 104. Mandible (Jaw) sions are of course present, but thev 



of Book-Louse. J 



seem like two, as the hinder two are 

 united into a single piece, which is, however, quite 

 separate from and movable upon the prothorax, which 

 in its turn forms a kind of narrow collar stretching 

 across just behind the head. The legs are attached to 

 the three divisions beneath, one pair to each ; they are 

 remarkable for the thickness of the thighs, which in the 

 hind pair are extraordinarily broad, presenting an appear- 

 ance such as usually indicates the possession of leaping 



